The main disinfectant in the drinking water of nearly 1 million D.C. and Northern Virginia residents is being switched by the Army Corps of Engineers to thwart the threat of terrorists releasing deadly chlorine gas.

The switch will be from chlorine gas to a liquid form of chlorine called sodium hypochlorite. Both are equally effective, according to the Washington Aqueduct, an arm of the U.S. Army Corps of Engineers. But the liquid, "is considered much safer to transport, store and use than gaseous chlorine," said an official.

Chlorine and water disinfection "may be the best thing to happen to the world" in the last 100 years, Thomas Jacobus, Washington Aqueduct general manager, told The Examiner. But the gaseous chlorine that currently is being used is potentially deadly if released; it was used in World War I as a choking agent.

"If you've got individuals or movements who want to try to use your own products againt you, if someone were to intercept a rail car, reroute it and release its contents, it could be devastating," Jacobus said.

The aqueduct provides roughly 180 million gallons of drinking water a day to about 1 million residents in the District, Arlington and Falls Church.

The switch is "absolutely a good sign" for homeland security in the nation's capital, said D.C. Councilman Phil Mendelson, chairman of the public safety committee.

But a broader fear remains: Dangerous materials are regularly transported through the District by rail, Mendelson said, and the federal government is far too secretive with that information.

"It appears that the shipments of the most hazardous materials have been removed from the Virginia Avenue [rail] line but they continue on the Northeast line," Mendelson said. "We just don't know."

The U.S. Environmental Protection Agency approved the chlorine swap in mid-December. Changeover presents no hazard, officials said. But the corps and the D.C. Water and Sewer Authority will nevertheless increase monitoring and testing for 18 months.

The change to liquid chlorine will start in the next month, Jacobus said. At the same time, the aqueduct also will begin adding caustic soda and a sulfuric acid solution to the water to balance pH levels.

Officials say city and Northern Virginia residents will not notice any difference in the way local water tastes or smells.

Washington Aqueduct water is disinfected in two stages -- the primary phase features chlorine and the second chloramine, a combination of chlorine and ammonia. Fluoride is added to reduce tooth decay, orthophosphate to control pipe corrosion and minimize lead release, and occasionally powdered activated carbon for taste and odor control.